The clinical manifestations of ulcerative colitis and Crohn's disease share the common feature of inflammation. In ulcerative colitis the earliest lesion is an inflammatory infiltration with abscess formation at the base of the crypts of Lieberkuhn. Coalescence of these distended and ruptured crypts tends to separate the overlying mucosa from its blood supply, leading to ulceration. The inflammatory involvement is diffuse and superficial, usually limited to the mucosa and submucosa.
The clinical picture includes cramping, lower abdominal pain or rectal bleeding, soon followed by frequent, loose discharges consisting mainly of blood, pus, and mucus with scanty fecal particles. The rectum and ampulla are usually found to be spastic.
In Crohn's disease (also known as regional enteritis or ulcerative ileitis) the most prominent feature of the disease is the granular, reddish-purple, edematous thickening of the bowel wall. In the early phase of the disease, the prominent irritability, spasm, and edema give the appearance of a rigid contour to the diseased segment radiogenographically.
The histological picture consists of dilated and tortuous lymph vessels and granulomatous structures which are made up predominantly of epithelioid cells, lymphocytes, and occasionally giant cells. With the development of inflammation, these granulomas often lose their circumscribed borders and merge with the surrounding tissue reaction. Obstruction is the predominant clinical feature. The stools although loose are rarely bloody.
Numerous mediators of inflammation have been demonstrated in the diseased tissue of inflammatory bowel disease. Among these are the prostaglandins and leukotrienes. Previous investigators have concluded that the inhibition of prostaglandin synthesis by cyclooxygenase inhibitors is either ineffective or contra-indicated for the treatment of inflammatory bowel disease. In contrast, the inhibition of leukotriene synthesis appears to be clinically beneficial.
Leukotrienes belong to a family of biomolecules called eicosanoids. Eicosanoids are derived from arachidonic acid and are classified according to their derivative enzymatic pathways, for instance, cyclooxygenase products which include prostaglandins, thromboxanes and prostacyclins; 15-lipoxygenase products, which include lipoxins; and 5-lipoxygenase products, which include the leukotrienes (LTA4, LTB4, LTC4, LTD4, and LTE4).
It has been known that leukotrienes have a role in allergic and inflammatory diseases. For instance, LTC4, LTD4 and LTE4 were previously referred to collectively as the slow-reacting substance of anaphylaxis (Morris et al., 1980, Nature, 285:104-106). Leukotrienes affect microvascular function by increasing hemodynamic margination of leukocytes, promoting permeability at postcapillary venules and eliciting leukocyte diapedesis, all leading to focal formation of interstitial inflammation.
Extravascular infiltration begins with adherence of leukocytes to the postcapillary venular endothelium. LTB4 and LTC4 can increase the adhesiveness of leukocytes to endothelial cells. LTC4, LTD4 and LTE4 can contract adjacent endothelial cells which increases permeability of venules.
LTB4 effects neutrophil chemotaxis, chemokinesis, aggegation, lysozyme release, superoxide production and augmentation of complement-dependent cytotoxicity. LTB4 also effects eosinophil, macrophage and monocyte chemotaxis and chemokinesis, and in lymphocytes increases cytotoxicity.
Evidence for the involvement of leukotrienes in the pathology of inflammatory bowel disease has accumulated over the last several years. For example, the ability of the mucosal tissue to synthesize LTB4 and the peptidoleukotrienes (LTC4, LTD4, and LTE4) are substantially elevated in both Crohn's disease and in ulcerative colitis (Sharon et al., 1984, Gastroenterol, 86:453-460; Dreyling et al., 1986, Biochim Biophys Acta, 878:184-193; Peskar et al., 1986, Agents Actions, 18:381-383).
Moreover, a positive correlation exists between the luminal concentration of LTB4 and the disease activity judged by clinical, endoscopic or histological gradings in ulcerative colitis (Lauritsen, et al., 1989, Gastroenterol, 91:837-844). The biological activities of the leukotrienes also support the hypothesis for a role in inflammatory bowel disease. The peptidoleukotrienes cause vascular permeability which may contribute to the tissue edema seen in both Crohn's disease and in ulcerative colitis (Hua, et al., 1985, Naunyn-Schmeideberg's Arch Pharmacol., 330:136-141).
Peptidoleukotrienes also contract intestinal smooth muscle (Cristol et al., 1988, Res Commun Chem Pathol Pharmacol., 59:423-426) and stimulate ion secretion from the ileum (Smith et al., 1988, Am J Physiol., 255:G175-183) and colonic mucosa (Jett et al., 1991, J Pharmacol Exp Therap. 257:698-705), which may contribute to the diarrhea associated with the intestinal inflammation.
Finally, LTB4 is the major chemotactic agent for neutrophils and macrophages found in the mucosal extracts of ulcerative colitis patients (Lobos et al., 1987, Dig. Dis. Sci., 32:1380-1388), which could account for the cellular infiltration.